Astrocytes have been proposed to play critical roles in plasticity as well as neuronal degeneration. Despite their abundance and key homeostatic roles very little is known regarding how astrocytes are generated and the mechanisms whereby astrocytes achieve specialized functions. In contrast, there are over 130 sub-classes of neurons and the transcriptional network regarding many of these lineages have been discovered. This is in part due to the significant emphasis that has been placed on understanding neurons but also due to the number of molecular and genetic markers that have been identified for different classes of neurons. The latter has facilitated the development of lineage specific tools and animal models to decipher the various roles of neuronal subtypes in divergent behaviors such as cognition, locomotion and sensory function. Here we propose to utilize a phage display screening approach to identify unique molecular aspects of astrocytes and astrocyte subtypes in order to accelerate our understanding of astrocyte function in health and disease. Cell-based phage screening is a powerful technology that involves the selection of peptide ligands from over 2 billion unique candidate peptide sequences based on desired cell binding properties and without a priori knowledge of specific receptors. We will combine our expertise in gliogenesis and glial cell biology with that of bioengineering, drug screening and discovery to develop novel tools for subclassifying astrocytes and also for direct imaging of astrocytes in vivo. We propose: Aim 1. Identification of astrocyte-specific peptides by phage display library selection, Aim 2. Evaluation of astrocyte-specific peptides as cell-specific markers for astrocytic populations and Aim 3. Development of a molecular probe for in vivo imaging of astrocytes. This proposal takes advantage of the unique research capabilities of the two principle investigators and their long-standing collaborative history. We expect that the tools and technologies developed through this proposed project will enable rapid advances in glial biology through the identification of unique biomarkers of astrocytes in general as well as astrocyte subtypes classified by function.